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Biological Membranes

• (^) Membranes are

found:

• (^) Separating

cells from their

external

environment

• (^) Separating

organelles

from the

cellular

environment

Partially permeable barriers

• (^) Partially permeable – only certain substances can pass through
• (^) Permeable – all substances can pass through

Role of membranes within cells

Membranes around cells (surrounding organelles):

• (^) Separate the organelle from the cytoplasm
• (^) Provide surfaces for metabolic reactions e.g. folded inner membranes of mitochondria is where aerobic respiration occurs
• (^) May contain enzymes that catalyse metabolic reactions, such as ATP synthase

Phospholipid Bilayer Basic structural component of plasma membranes Consists of:

• (^) Two layers of phospholipids
• (^) Proteins embedded in the phospholipids
• (^) Cholesterol within the tails of the phospholipids

Phospholipids

Phosphate and glycerol head Fatty acid tails

Polar-

makes

head

hydrophili

c

“Water loving”

Non-polar-

makes tails

hydrophobic

“Water hating”

Phospholipid bilayer

• (^) When exposed to water (like on the inside and outside of cells) phospholipids form a double layer (a bilayer) Keeps the hydrophilic heads near to water (they attract) Keeps the hydrophobic tails away from water (they repel)

Fluid Mosaic Model of Membrane

Structure

• (^) There are also proteins and cholesterol in the phospholipid bilayer – this forms the fluid mosaic model of membrane structure
• (^) Features:
• (^) Phospholipid bilayer
• (^) Proteins embedded- Proteins can move laterally in the membrane (side to side, not across from one side to another)

B A C D phospholip id bilayer Integral or intrinsi c protein Periphera l or extrinsic Cholester^ protein ol Hydrophili c head Hydrophobic tail

Types of protein- intrinsic proteins

• (^) Intrinsic proteins (integral proteins) are embedded through both layers of phospholipids
• (^) Channel proteins and carrier proteins are both intrinsic proteins

Carrier Proteins- Used for facilitated

diffusion or active transport

• (^) Provide pores for large molecules to move through to enter or exit the cell
• (^) Examples include glucose and amino acids
• (^) They are shaped to only allow certain molecules to bind to them
• (^) Once the molecule has bound, the carrier protein changes shape to transfer it across the membrane

Receptors

• (^) Cells have proteins embedded in their surface membrane called receptors
• (^) Signalling molecules such as hormones can bind to these receptors
• (^) The signalling molecules can bind to the receptors because they have a shape which is complementary to the shape of the receptor

Example of a receptor

• (^) β-cells of the pancreas release the hormone insulin. Insulin travels in the blood and binds to receptors on muscle cells. This triggers a response and causes them to increase uptake of glucose

Cell Recognition and

Identification

Antigens- for cell recognition and Identification. These can be extrinsic or peripheral proteins (that are only present on one side of the bilayer). They can also be glycolipids. Antigens

• (^) Have a specific shape
• (^) Are what the immune cells recognise as “self” or “non-self”